car rtc on a motorway advice needed please

hi all, tonight i was driving along a motorway as i have come round a bend near a off slip a car has slammed its breaks on and stopped dead! on the off slip rd on the motorway i could not go round him as cars in other lanes so i ended up going into the back of his car almost underneath as i breaked that hard all i have damaged is bottom of his bumper but my car has folded up on the front end as its all plastic wings and bonnet. now where do i stand he said infront of the police that the car had been playing up kept on cutting out and this time it cut out but instead of him rolling into a hard shoulder he slammed on his brakes and i went into the back of him. surely if you know your car is not running right you would not risk driving it on a motorway and to just slam on your brakes is crazy. i was only doing 40-45 as like i said i was joining the off slip. the even worse news for me is that i am only 3r party and this other guy is driving his wifes car but on his insurance which is for a different car. any advi e please as i am so wooried all this is going to effect me getting into work as i have no car to drive but i dont think it was my fault even though everybody says that but in this case slamming your brakes on a motorway doesnt leave much hope for anyone behind you. something else i forgot to mention, i heard on the police radio that it was a lady registered to drive the car i hit but a man was driving it. he said he is insured to drive the car but his insurance is on a different car/reg, does that sound right as i know people think you can jump in cars and drive others on your insurance but like mine nobody is entitled to drive my car neither am i to drive other peoples car. i also did not admit liability as i think what he done was dangerous.